The present invention relates to an implant device to be placed into a portion of the intervertebral space between adjacent vertebrae. Specifically, the invention concerns an expandable osteogenic fusion device that may enhance arthrodesis or fusion between adjacent vertebral bodies while also maintaining the height of the intervertebral space at the instrumented vertebral level.
In many cases, low back pain originates from damages or defects in a spinal disc between adjacent vertebral bodies. The disc can be herniated or can be affected by a variety of degenerative conditions. Frequently, pathologies affecting the spinal disc can disrupt the normal anatomical function of the disc. In some cases, this disruption is significant enough that surgical intervention is indicated.
In one such surgical treatment, the affected disc is essentially removed and the adjacent vertebral bodies are fused together. In this treatment, a discectomy procedure is conducted to remove the disc nucleus while retaining the annulus. Since the disc material has been removed, an implant must be placed within the intervertebral space to prevent the space from collapsing.
In early spinal fusion techniques, bone material, or bone osteogenic fusion devices, were simply disposed between adjacent vertebral bodies, typically at the posterior aspect of the vertebral bodies. In the early history of these bone osteogenic fusion devices, the devices were formed of cortical-cancellous bone which was generally not strong enough to support the weight of the spinal column at the instrumented level. Consequently, the spine was stabilized by way of a plate or a rod spanning the affected vertebral bodies. With this technique, once fusion occurred across the vertebral bodies and incorporated the bone osteogenic fusion device, the hardware used to maintain the stability of the spine became superfluous.
Following the successes of the early fusion techniques, focus was directed to modifying the device placed within the intervertebral space. Attention was then turned to implants, or interbody fusion devices, that could be interposed between the adjacent vertebral bodies, maintain the stability of the disc interspace, and still permit bone fusion or arthrodesis. These interbody fusion devices have taken many forms. For example, one prevalent form is a cylindrical hollow implant or “cage”. The outer wall of the cage creates an interior space within the cylindrical implant that is filled with, for example, bone chips or other bone growth-inducing material. In recent years compounds known as bone morphogenetic proteins (BMPs) have become the preferred bone growth inducing material. In some cases, the cylindrical implants included a threaded exterior to permit threaded insertion into a tapped bore formed in portions of the adjacent vertebral bodies. Alternatively, some fusion implants have been designed to be impacted into the intervertebral space. Yet another class of fusion implants can be placed in between adjacent vertebral bodies and then expanded to contact the opposing surfaces of the vertebral bodies.
Experience with some interbody fusion devices has demonstrated the efficacy of some such implants in yielding a solid bone fusion. Variations in the design of the implants have accounted for improvements in stabilizing the motion segment while fusion occurs. Nevertheless, with some of the interbody fusion devices, there remains difficulty in achieving a complete fusion, at least without the aid of some additional stabilizing device, such as a rod or plate. Moreover, some of the devices are not structurally strong enough to support some loads and bending moments applied at certain levels of the spine.
Further difficulty has been encountered when a surgeon, desiring to avoid removal of the spinal facet joints laterally, uses an undersized interbody fusion cage in a posterior lumbar interbody fusion procedure (PLIF). Use of undersized devices results in sub-optimal contact with the endplates of adjacent vertebral bodies and consequent sub-optimal bone formation inside the device, and can lead to pseudoarthrosis. Additionally, undersized devices may not provide adequate disc space distraction and nerve root decompression. Due to the high degree of anatomical and physiological variation encountered in all surgery, efforts to avoid utilization of a posteriorly undersized implant can require the availability of numerous devices of different dimensions, and increase the time required to carry out the surgical procedure, thus increasing the cost and risk associated with the procedure. Some prior efforts to address this difficulty through use of expandable devices have utilized designs involving numerous parts, or designs that apply excessive stress force to the device, resulting in device strain. These design approaches increase the risk of mechanical failure. Also, they may occlude the space between vertebral body endplates, inhibiting fusion from adequately occurring.
Even with devices that do not have the aforementioned difficulties, still other undesirable characteristics exist. Studies have suggested that the interbody fusion implant devices, especially those implants of the “cage” design, lead to stress-shielding of bone material within the cage. It is well known that bone growth is enhanced by stressing or loading the bone material. The stress-shielding phenomenon relieves some or all of the load applied to the bone material to be fused, which can greatly increase the time for complete bone fusion, or disturb the quality and density of the ultimately formed fusion mass. In some instances, stress-shielding can cause the bone chips or fusion mass contained within the fusion cage to resorb or evolve into fibrous tissue rather than into a bony fusion mass. A further difficulty encountered with many fusion implants is that the material of the implant is not radiolucent. Most fusion cages are formed of metal, such as stainless steel, titanium or porous tantalum. The metal of the cage shows up prominently in any radiograph (x-ray) or computer tomography (CT) scan. Since “cage” type fusion devices surround and contain the bone graft material housed within a metal cage, the developing fusion mass within the cage cannot be seen under traditional radiographic visualizing techniques, and can be seen in CT scans only with the assistance of image scatter techniques. Thus, the spinal surgeon does not have adequate means to determine the progress of the fusion, and in some cases cannot ascertain whether the fusion was complete and successful.
Thus, the field of spinal fusion lacks a suitable intervertebral fusion device that can be made small enough to facilitate insertion in the intervertebral space and support bone growth material within the intervertebral space and expand to maintain the normal height of the disc space. Further, current spinal fusion devices do not sufficiently reduce the risk of stress-shielding the fusion mass and do not enable visualization of the fusion mass as the arthrodesis progresses. So, there remains a need for improvements in osteogenic fusion device technology, particularly devices that provide expandable characteristics. The present invention addresses this need in a novel and non-obvious fashion.